All NAND flash-based solid-state drives, which are the majority of SSDs on the market, have a finite time of writing, called "write endurance rating." In other words, an SSD can be written on a limited number of times before it becomes unreliable. Of course, before you can rewrite on a part of the drive, you'll need to first erase information already stored on that part. This is why the write endurance rating is also known as the P/E cycle, or program/erase cycle.

Storage talk: Understanding your solid-state drive